1. Field of the Invention
The present invention relates to an image forming apparatus comprising means for automatically controlling the density of a formed image.
The present invention is also effective for use in systems such as copying systems, laser-beam printers, printing machines and the like in which an image is to be formed on an image bearing member having a photosensitive layer in accordance with image information from an original object such as a document and which comprises an automatic image density controlling mechanism having means for detecting optical or potential information corresponding to the image information.
2. Description of the Prior Art
In the conventional imge forming systems such as copying machines and others, an automatic adjustment has been carried out to make an image clearer by controlling its density even if there are different densities for different images. This automatic adjustment is convenient and more reliable than manual adjustment, when originals of different densities are to be copied.
Even if there is such an advantage, the prior art image forming systems cannot avoid improper functions or malfunctions due to some change of the preset process conditions with the passing of time and due to the contamination of optical components such as lenses, mirrors and others.
Japanese Laid-open Patent Application No. 141646/1979 discloses means for overcoming the malfunction based on the change of the preset process conditions. A measurement is compared with a reference value to control the output of an original exposure lamp, the development bias of a developing device or others. However, such comparison and control is time-consuming. In a high-speed copying machine, images may be processed before the comparison and control of the value is completed, so that an entirely proper image cannot be obtained.
U.S. Pat. No. 4,215,930 discloses means for overcoming the malfunction based on the contamination of the lenses and mirrors. This proposal utilizes the same method as in the aforementioned Japanese Laid-open Patent Application for controlling the potential of a lamp, the width of a slit through which an light image can be transmitted, the charge to a photosensitive member and so on. Therefore, this U.S. Patent device also has the same problem.
It is also known in the art to program or incorporate image formation conditions into a circuitry for effecting a preselected calculation on the premise that means for detecting the potential or the amount of light is stable in sensitivity. This will be called "image formation characteristics". In accordance with the detected conditions of image density from a source of information, proper image formation conditions can normally be obtained from the above image formation characteristics.
However, if the means for detecting the potential or the amount of light as information of image density is varied in sensitivity for any reason such as the deposition of toner, the deposition of dust or the reduction of its own sensitivity, the automatic image density adjusting system will malfunction repeatedly so that no proper image will be obtained. Such a problem cannot be overcome by the above described prior art.